Process for wet passivation of bond pads for protection against subsequent TMAH-based processing

ABSTRACT

A process for forming a protective layer at a surface of an aluminum bond pad. The aluminum bond pad is exposed to a solution containing silicon, ammonium persulfate and tetramethylammonium hydroxide, which results in the formation of the protective layer. This protective layer protects the bond pad surface from corrosion during processing of an imager, such as during formation of a color filter array or a micro-lens array.

This application is a division of U.S. patent application Ser. No.12/390,012, filed Feb. 20, 2009 U.S. Pat. No. 8,198,731, which claimspriority under 35 U.S.C. 119 to Italian patent application No.RM2008A000610, filed Nov. 13, 2008, which are hereby incorporated byreference in their entireties.

FIELD OF THE INVENTION

Disclosed embodiments relate generally to semiconductor circuits, andmore particularly to a process for forming a protective layer on thesurface of aluminum bond pads that protects the surface of the bond padsduring subsequent processing.

BACKGROUND OF THE INVENTION

In semiconductor circuits and devices, bond pads are used to makeelectrical contact between various components of the circuit or device.For simplicity of discussion, the following discussion focuses on bondpads for imagers; however, the scope of the invention should not belimited as such and may be used for any type of integrated circuitdevice.

Imagers, including charge coupled devices (CCDs), photodiode arrays,charge injection devices (CIDs), hybrid focal plane arrays, andcomplementary metal oxide semiconductor (CMOS) imagers, have commonlybeen used in photo-imaging applications. Current applications ofsolid-state imagers include cameras, scanners, machine vision systems,vehicle navigation systems, star trackers, and motion detector systems,among others.

A CMOS imager typically includes a focal plane array of pixel cells,each one of the cells including a photosensor, for example, a photogate,photoconductor or a photodiode for accumulating photo-generated charge.Each pixel cell has a charge storage region, which is connected to thegate of an output transistor that is part of a readout circuit. Thecharge storage region may be constructed as a floating diffusion region.In some imagers, each pixel cell may include at least one electronicdevice such as a transistor for transferring charge from the photosensorto the storage region and one device, also typically a transistor, forresetting the storage region to a predetermined charge level prior tocharge transference.

To allow the photosensors to capture a color image, the photosensorsmust be able to separately detect color components of the capturedimage. Accordingly, each pixel must be sensitive only to one color orspectral band. For this, a color filter array (CFA) is typically placedin front of the optical path to the photosensors so that eachphotosensor detects the light of the color of its associated filter.

Further, since the magnitude of the signal produced by each pixel isproportional to the amount of light impinging on the photosensor, it isalso desirable to improve the photosensitivity of the imager bycollecting light from a large light collecting area and focusing it ontoa small photosensitive area of the photosensor. This can be done using amicro-lens array formed over the pixel array.

FIGS. 1A and 1B illustrate an imager 10 including a color filter array30 and micro-lens array 35 over a pixel array 25. As can be seen in FIG.1A, imaging portion 15 may be surrounded by peripheral circuitry 18 andbond pads 20. The bond pads 20 are electrically connected to devicecircuitry of the imager 10. Peripheral circuitry 18 controls the imagingportion 15 and converts electrical signals received from the imagingportion 15 into a digital image. As can be seen in FIG. 1B, the pixelarray 25, peripheral circuitry 18 and bond pads 20 are formed insemiconductor wafer 50. Other conventional parts of the imager 10 arenot shown or described herein.

The process for forming the color filter array 30 and/or micro-lensarray 35 over the pixel array 25 requires a multi-step fabricationprocess. A common method of forming these structures includes atetramethylammonium hydroxide (TMAH)-based developing solution.Typically, TMAH-based developing solutions used for color filter arrayformation have a TMAH concentration ranging from about 0.6% to about2.6%. The entire CFA/microlens formation process typically involvesabout six developing steps where this developing solution comes intocontact with the bond pad for approximately 30 seconds to 1 minute pereach step. The TMAH contained in this developing solution is veryaggressive on the aluminum from which the bond pads 20 are typicallyformed. As can be seen in FIG. 2, when an aluminum bond pad 20 isexposed to a TMAH-based developing solution 55, it is attacked andslowly etched away, creating pits 60 on the surface of the aluminum,making the surface very rough. Through an entire CFA/microlens formationprocess, about 100-200 nm of aluminum may be removed from the surface ofan unprotected bondpad. This pitting and removal of aluminum can causeproblems in later processing and use of the imager. The roughness,caused by the pitting, increases the scattering of light and makes thepads look very dark when viewing the pads through a scope. In addition,the excessive removal of aluminum can cause bonding issues, caused bytoo much topography in the aluminum layer or by not enough aluminumremaining in the bondpad.

An organic protective layer or an oxide layer can be formed over thebond pads 20 before the CFA/micro-lens formation process to protect thebond pads 20 from the TMAH solution. However, these layers must beremoved after the CFA/micro-lens formation process in order to avoidproblems (e.g., bonding issues during packaging of the imager) duringlater processing steps. Furthermore, the oxide layer, for example, mustbe removed using an expensive hard-coat process.

Accordingly, there is a need and desire for a simpler process by whichbond pads may be protected during subsequent processing of asemiconductor device, for example, an imager.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic version of a top view of a conventional imager.

FIG. 1B is a schematic cross-sectional side view of the imager of FIG.1A.

FIG. 2 illustrates the effect of a TMAH-based developing solution on anunprotected aluminum bond pad.

FIG. 3 illustrates a process for forming a protective Al—Si—O layer onbond pads, in accordance with disclosed embodiments.

FIG. 4 illustrates experimental results of the silicon depth profile ofeight bond pads processed with the disclosed process.

FIG. 5 illustrates experimental results of the oxygen depth profile ofeight bond pads processed with the disclosed process.

FIG. 6 illustrates experimental results of the aluminum depth profile ofeight bond pads processed with the disclosed process.

FIG. 7 illustrates the effect of a TMAH-based developing solution on aprotected aluminum bond pad, in accordance with disclosed embodiments.

FIG. 8 illustrates an element composition depth profile of an aluminumbond pad processed in accordance with the disclosed process and througha color filter array process.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which are shownby way of illustration specific embodiments that may be practiced. Itshould be understood that like reference numbers represent like elementsthroughout the drawings. These example embodiments are described insufficient detail to enable those skilled in the art to practice them.It is to be understood that other embodiments may be utilized, and thatstructural, material, and electrical changes may be made, only some ofwhich are discussed in detail below.

Disclosed embodiments relate to a process performed on an imager beforeformation of the color filter array and/or micro-lens array, though, asnoted, the process can be used with bond pads of other semiconductordevices and circuits.

The disclosed process is able to effectively protect the aluminum bondpads from unwanted damage typically caused by a TMAH-based developingprocess used in the CFA/micro-lens formation process. Before formationof the CFA or micro-lens array, a wet process is performed that createsa very thin layer of an aluminum-silicon-oxygen (Al_(x)Si_(x)O_(x))compound on the surface of the bond pad. The presence of the silicon inthis thin layer of Al_(x)Si_(x)O_(x) at the surface of the bond padmakes the aluminum from which the bond pad is formed resistant to theTMAH.

FIG. 3 illustrates the wet process by which the Al—Si—O compound isformed on the surface of a bond pad 20 a. The bond pad 20 a is exposedin a wet process to a solution 65 comprising deionized water, silicon(Si), ammonium persulfate (AP) and TMAH (TMA⁺ and OH⁻) and having a pHof 12 +/−0.1. During exposure to the solution 65, silicon and oxygen areincorporated into the surface of the aluminum bond pad 20 a, therebyforming a thin layer of an Al_(x)Si_(x)O_(x) compound 70 and resultingin a protected bond pad 20 b. The solution 65 comprises between about3.0% and about 3.4% silicon, between about 2.2% and about 2.8% ammoniumpersulfate and between about 6.5% and about 8.5% TMA⁺ ions. The bond pad20 a is exposed to the solution 65, followed by a DI water rinse and aMarangoni rinse, which dries the wafer.

While the exact mechanism of reaction for the formation of theprotective layer is not known, it is thought that the solution has anionic interaction with the aluminum on the surface of the bond pad. Somealuminum atoms are brought into the solution and substituted with thesilicon atoms, thereby forming a more resistant bond on the surface ofthe bond pad. This interaction stops when the surface has been uniformlyimplanted with the maximum amount silicon with respect to thetemperature and pH at which it is being processed.

In one embodiment, the process solution 65 comprises deionized watercontaining about 3.2% silicon, about 2.5% ammonium persulfate and about7.5% TMA⁺ and has a pH of 12 +/−0.1. The bond pad 20 a is exposed to thesolution 65 for about 6 minutes, maintaining the temperature at about90° C., followed by a DI water rinse at about 40° C. and a Marangonirinse, which dries the wafer. It may also be possible to expose the bondpad to the process solution 65 at temperatures as low as about 60° C.The variation in temperature results in a variation in concentration ofsilicon in the protective layer.

The thin layer of Al—Si—O compound 70 may have a thickness of up toabout 3.5 nm. Preferably the thickness is about 2 nm. This thickness issufficient to protect the bond pad 20 b from attack by TMAH solutionused in subsequent processing, such as during formation of a CFA and/ormicro-lens array, but thin enough to allow uninhibited wire bonding atlater processing steps (without needing to remove the protective layer).

The Al_(x)Si_(x)O_(x) compound 70 formed at an upper portion of the bondpad 20 b contains between about 4% and about 18% silicon, between about40% and about 60% oxygen and between about 5% and about 30% aluminum.The balance of the is small amounts of other elements, such as carbon,nitrogen, fluorine and sulfur present in the bond pad 20 a beforeprocessing. In one embodiment, the Al_(x)Si_(x)O_(x) compound 70 formedat an upper portion of the bond pad 20 b contains about 15% silicon,about 50% oxygen and about 20% aluminum.

FIG. 4 illustrates experimental results of the silicon depth profile ofeight bond pads processed with the disclosed process. FIG. 5 illustratesexperimental results of the oxygen depth profile of eight bond padsprocessed with the disclosed process. FIG. 6 illustrates experimentalresults of the aluminum depth profile of eight bond pads processed withthe disclosed process. FIGS. 4-6 illustrate the concentration ofsilicon, oxygen and aluminum, respectively, at increasing depths belowthe surface of the bond pad. These charts show the extent ofincorporation of silicon and oxygen into the surface of the aluminumbond pads by using the claimed process. These depth profiles werecreated based on an X-ray Photoelectron Spectroscopy (XPS) analysis ofthe bondpad surfaces.

After the disclosed process is performed, the wafers can be subjected tosubsequent processing, such as CFA/micro-lens formation using astandard, known process, without corrosion or pitting occurring on thebond pads. Additionally, the protective layer does not need to beremoved prior to the subsequent processing. As can be seen in FIG. 7,when the protected aluminum bond pad 20 b is exposed to a TMAH-baseddeveloping solution 55, aluminum is not removed from the surface of thepad 20 b. Therefore, the problems associated with the pitting seen inFIG. 2 are avoided.

FIG. 8 illustrates a typical element composition depth profile of analuminum bond pad processed with the wet solution in accordance withdisclosed embodiments and then through a color filter array formationprocess. Again, these depth profiles were created based on an XPSanalysis of the bondpad surfaces. FIG. 8 demonstrates that theprotective layer (circled region on FIG. 8) is able to resist the reworkprocess performed in the CFA process to allow reprocessing of thewafers. This means that this layer can easily stand multipleCFA/micro-lens processes without the necessity to reapply it.

While disclosed embodiments have been described in detail, it should bereadily understood that the claimed invention is not limited to thedisclosed embodiments or to use with imagers. Rather the disclosedembodiments can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed.

1. A method of protecting an aluminum bond pad from subsequentprocessing steps, comprising: forming a protective layer on the aluminumbond pad by exposing the aluminum bond pad to a solution having siliconand tetramethylammonium hydroxide; and exposing the aluminum bond pad toa developing solution having tetramethylammonium hydroxide during asubsequent processing step, wherein the protective layer protects thealuminum bond pad from the developing solution.
 2. The method defined inclaim 1, wherein forming the protective layer on the aluminum bond padcomprises forming a protective layer comprising aluminum, silicon, andoxygen on the aluminum bond pad.
 3. The method defined in claim 1,wherein the aluminum bond pad is located on an imager having a pixelarray and wherein exposing the aluminum bond pad to the developingsolution having tetramethylammonium hydroxide during the subsequentprocessing step comprises exposing the aluminum bond pad to thedeveloping solution having tetramethylammonium hydroxide duringformation of a color filter array over the pixel array.
 4. The methoddefined in claim 1, wherein the aluminum bond pad is located on animager having a pixel array and wherein exposing the aluminum bond padto the developing solution having tetramethylammonium hydroxide duringthe subsequent processing step comprises exposing the aluminum bond padto the developing solution having tetramethylammonium hydroxide duringformation of a micro-lens array over the pixel array.
 5. The methoddefined in claim 1, wherein the aluminum bond pad is located on animager having a pixel array and wherein exposing the aluminum bond padto the developing solution having tetramethylammonium hydroxide duringthe subsequent processing step comprises exposing the aluminum bond padto the developing solution having tetramethylammonium hydroxide duringone of a reworking of a micro-lens array over the pixel array and areworking of a color filter array over the pixel array.
 6. The methoddefined in claim 1, further comprising wire-bonding a wire to thealuminum bond pad without removing the protective layer, wherein theprotective layer has a thickness of up to about 3.5 nm.
 7. The methoddefined in claim 1, further comprising wire-bonding a wire to thealuminum bond pad without removing the protective layer, wherein theprotective layer has a thickness of about 2 nm.
 8. A method ofprotecting an aluminum bond pad from subsequent processing steps,comprising: forming a protective layer on the aluminum bond pad byexposing the aluminum bond pad to a solution having silicon andtetramethylammonium hydroxide, wherein the protective layer comprisesaluminum, silicon, and oxygen.
 9. The method defined in claim 8, furthercomprising: exposing the aluminum bond pad to a developing solutionduring a plurality of subsequent processing steps, wherein theprotective layer protects the aluminum bond pad from the developingsolution, wherein the plurality of subsequent processing steps include aplurality of reworking steps of one of a color filter array and amicro-lens array.
 10. The method defined in claim 8, further comprising:exposing the aluminum bond pad to a developing solution havingtetramethylammonium hydroxide during a plurality of subsequentprocessing steps, wherein the protective layer protects the aluminumbond pad from the developing solution.
 11. The method defined in claim8, wherein forming the protective layer on the aluminum bond pad byexposing the aluminum bond pad to the solution having silicon andtetramethylammonium hydroxide comprises forming the protective layer onthe aluminum bond pad by exposing the aluminum bond pad to a solutionhaving silicon and tetramethylammonium hydroxide, and having a pH of12+/−0.1.
 12. The method defined in claim 8, wherein forming theprotective layer on the aluminum bond pad by exposing the aluminum bondpad to the solution having silicon and tetramethylammonium hydroxidecomprises forming the protective layer on the aluminum bond pad byexposing the aluminum bond pad to a solution having between about 6.5%and about 8.5% tetramethylammonium ions.
 13. The method defined in claim8, wherein forming the protective layer on the aluminum bond pad byexposing the aluminum bond pad to the solution having silicon andtetramethylammonium hydroxide comprises forming the protective layer onthe aluminum bond pad by exposing the aluminum bond pad to a solutionhaving between about 3.0% and about 3.4% silicon.
 14. The method definedin claim 8, wherein forming the protective layer on the aluminum bondpad by exposing the aluminum bond pad to the solution having silicon andtetramethylammonium hydroxide comprises forming the protective layer onthe aluminum bond pad by exposing the aluminum bond pad to a solutionhaving about 3.2% silicon and about 7.5% tetramethylammonium ions.
 15. Amethod of protecting an aluminum bond pad from subsequent processingsteps, comprising: forming a protective layer on the aluminum bond padby a wet process, wherein the protective layer is formed from aluminum,silicon, and oxygen and wherein the aluminum in the protective layer isincorporated from the aluminum bond pad.
 16. The method defined in claim15, further comprising: exposing the aluminum bond pad to a developingsolution having tetramethylammonium hydroxide during a subsequentprocessing step, wherein the protective layer protects the aluminum bondpad from the developing solution.
 17. The method defined in claim 15,wherein forming the protective layer on the aluminum bond pad by the wetprocess comprises forming the protective layer on the aluminum bond padby exposing the aluminum bond pad to a solution havingtetramethylammonium hydroxide, wherein the protective layer comprisesbetween about 4% and about 18% silicon, between about 40% and about 60%oxygen and between about 5% and about 30% aluminum, and wherein thesolution also includes ammonium persulfate .
 18. The method defined inclaim 15, wherein forming the protective layer on the aluminum bond padby the wet process comprises forming the protective layer on thealuminum bond pad by exposing the aluminum bond pad to a solution havingtetramethylammonium hydroxide at a temperature between about 60° C. andabout 90° C.
 19. The method defined in claim 15, further comprising:exposing the aluminum bond pad to a developing solution havingtetramethylammonium hydroxide during a subsequent processing step,wherein the protective layer protects the aluminum bond pad from thedeveloping solution; and wire-bonding a wire to the aluminum bond padwithout removing the protective layer.
 20. The method defined in claim15, wherein the aluminum bond pad is located on an imager having a pixelarray, the method further comprising: exposing the aluminum bond pad toa developing solution having tetramethylammonium hydroxide during aformation of one of a color filter array and a micro-lens array, whereinthe protective layer protects the aluminum bond pad from the developingsolution; and wire-bonding a wire to the aluminum bond pad withoutremoving the protective layer.